1. Field of the Invention
The present invention relates to the treatment of formations surrounding oil wells, gas wells, injection wells and similar boreholes by the injection of a microemulsion containing an aqueous acid solution.
2. The Prior Art
Basically, all well treatments involve the injection of a fluid into an oil or gas well to stimulate production from the well. The most widely used stimulation technique is hydraulic fracturing, in which a fracturing fluid is injected into a well under pressure to propagate a fracture adjacent to the well. Another widely used stimulation method is acidizing, in which an acidic fluid is introduced into the formation to dissolve formation rock. One acidizing technique is known as matrix acidizing, whereby an acid is injected into the formation to etch into the pore spaces.
In matrix acidizing, the acid-bearing fluid is slowly injected into the formation so that the acid can permeate into channels that have been clogged or constricted with clays or formation fines. The acid dissolves the hindering material, thereby increasing permeability. Tests have shown that the degree of stimulation afforded by the matrix acidizing is strongly dependent upon the extent of acid penetration into the formation.
One method described in British Pat. No. 2 022 653 for providing deep acid penetration involves the use of an emulsion comprising an oil external phase and an acid internal phase. The external oil phase apparently shields the acid from the formation materials and thereby permits unreacted acid to penetrate deeper into the formation.
Relatively recently, microemulsions have been suggested as a means of transporting the acid into the formation. This seems to be an attractive approach since microemulsions are generally known to have very good transport properties into a variety of porous materials. One of the reasons for the good penetrating power of microemulsions is their ability to give ultralow interfacial tension with both oil and aqueous phases.
With microemulsions are here meant thermodynamically stable, isotropic solutions consisting of oil, water, surfactant system and optionally other ingredients. The surfactant system usually consists of two components, the "surfactant" which is a normal type of surface active compound and the "cosurfactant", which is normally of low molecular weight and not very surface active. The most commonly used cosurfactants are alcohols such as butanol and pentanol.
Microemulsions have originally been classified as water in oil (w/o) or oil in water (o/w) in a manner similar to what is used for macroemulsions. Recent investigations, however, have shown that many microemulsions are neither w/o nor o/w, but may best be characterized as bicontinuous.
Originally microemulsions based on anionic surfactants of the sulfonate type have been proposed in the U.S. Pat. No. 3,754,599 and U.S. Pat. No. 3,831,679. However, these microemulsions have the drawback that the surfactant reacts with the acid, forming the corresponding protonated species, i.e., the sulfonic acid. This leads to an unwanted consumption of acid, as well as to the risk of precipitation of the surfactant.
In the Britist patent application No. 2,074,043, a cationic amine surfactant is suggested as the base surfactant for making microemulsions of hydrocarbon oil and aqueous acid solution. In this case no reaction between the acid and the surfactant takes place. A cosurfactant, usually an alcohol, is also needed in an amount approximately equal to that of the surfactant.